The present invention relates to the production of food products for carnivorous animals, especially mink.
Conventional methods for producing dry animal food involve either extrusion or pelleting techniques wherein food materials are formed into the desired shape for acceptance by the animal. Dry animal food is commonly produced by extruding mixed, farinaceous-proteinaceous sources with nutritionally balancing vitamins and minerals and flavor factors. The mixed materials are tempered to a specific moisture range and then subjected to the action of an extruder-cooker which mixes the materials and subjects them to heat at high pressure to convert the materials to a flowable liquid substance. The temperature of the mixed liquid material in the extruder is usually above 212.degree. F. The heated liquid materials are expelled through the extruder die to atmospheric pressure, where the heated moisture flashes to steam causing the material to expand into a cellular mass. The cellular mass is then cut into particles, dried to a stable moisture content and then possibly sprayed with animal fat. Conventional extrusion methods however do not show the incorporation of high amounts of fat into the premix of ingredients or into the extruder for this hampers the operation of the extruder-cooker and tends to form a product that crumbles easily and lacks particle integrity. Usually, fat is applied to the food material upon extrusion. About 2 to about 12% fat can be applied to food material in this manner. See U.S. Pat. Nos. 3,365,297 and 3,115,409. Other processes such as that shown in U.S. Pat. No. 3,623,885 show the extrusion of food materials containing a small amount of fat. However, a binding agent such as .alpha.-cellulose or starch must be added to the extrusion mix prior to extrusion in order to bind the fat to the food materials. In this manner, the food materials are able to be cooked in the extruder and still retain some degree of particle integrity.
The other technique used for incorporating fat into animal feed concerns pelletizing. Simply, pelletizing involves the mixture of desired food materials, grinding and heating of the materials, and then forcing the materials through a die of a pellet mill. The amount of added fat in this operation is usually limited to less than 4% and the resultant product will have poor structural quality. Additionally, in more recent processes such as that described in U.S. Pat. No. 3,014,800 and U.S. Pat. No. 2,945,764 the manufacture of high fat pellets is shown by first mixing and comminuting the composite of ingredients, then pelletizing the ingredients, and after drying, applying a hot liquid fat to the surface of the pellets. If the pellets are not dried adequately prior to the application of the fat, the fat will be absorbed by the pellets and a soft product will result. The maximum amount of total fat which can be incorporated within and onto the pellet without causing undesirable crumbling and high percentage of fines is from about 10 to 20%. If too much fat is applied to the surface of the pellets, that is, over 20% by weight, the product will be soft, friable, crumbly and exhibit a greasy texture. Another recent process shown in U.S. Pat. No. 3,993,796 involves using a spray dried blend of protein isolate and edible oil which is mixed with the food ingredients prior to pelletizing.
However, it is desirable that animals such as mink whose nutritional requirements require high levels of fat per unit of diet be provided with a dry expanded product. In accordance therewith, it is highly desirable that an extruded feed containing high amounts of fat is provided in difference to the dry products produced generally by complex pelletizing operations.